The present invention relates to a circular polarizer connected to a primary radiator of a parabolic antenna sharing two frequency bands, and particularly to a circular polarizer provided at an outer waveguide for a low frequency band in waveguides of the coaxial structure connected to a primary radiator.
Recently, satellite broadcast receivers have become popular. In general, the polarized wave of a signal used in satellite broadcasting includes a circularly polarized wave in addition to a linearly polarized wave. FIG. 1 shows an example of an appearance of a parabolic antenna employed by a satellite broadcast received using the conventional circularly polarized wave. Referring to FIG. 1, the parabolic antenna includes a dish 51 reflecting a circularly polarized wave, a primary radiator 52 receiving the circularly polarized wave collected by dish 51, a circular polarizer 53 converting the circularly polarized wave received by primary radiator 52 into a linearly polarized wave, and a converter 54 converting the frequency of the linearly polarized wave output from circular polarizer 53. A circular polarizer is a polarized wave converter converting a linearly polarized wave into a circularly polarized wave, or a circularly polarized wave into a linearly polarized wave.
FIGS. 2A, 2B, 2C, 2D schematically show structures of conventional circular polarizers. These circular polarizers 53a, 53b, 53c and 53d, respectively convert a circularly polarized wave into a linearly polarized wave. The operation mechanism will be briefly described hereinafter.
In the case where a circularly polarized wave is to be converted into a linearly polarized wave, it is assumed that the two linearly polarized waves orthogonal to each other constitute the circularly polarized wave and the phases of the two linearly polarized waves are displaced by 90xc2x0. A circularly polarized wave Ec is converted into a linearly polarized wave Er by retarding the phase of the linearly polarized wave that is advanced 90xc2x0 to set the phase difference to 0xc2x0.
For example, a dielectric phase plate 61 in a circular polarizer 53a shown in FIG. 2A is provided to have an angle of approximately 45xc2x0 with respect to a linearly polarized wave Er that is to be converted. An electric field E1 parallel to dielectric phase plate 61 passes through dielectric phase plate 61, whereby the wavelength is reduced. As a result, the phase of electric field E1 is behind the phase of an electric field E2 orthogonal to dielectric phase plate 61. By setting this phase delay to 90xc2x0, the phase difference between electric fields E1 and E2 becomes 0xc2x0, whereby circularly polarized wave Ec can be converted into linearly polarized wave Er.
Circular polarizer 53b of FIG. 2B is provided with a plurality of cylindrical metal projections at the waveguide. By retarding the phase of electric field E1 90xc2x0 by the cylindrical metal projection, circularly polarized wave Ec is converted into linearly polarized wave Er. Circular polarizer 53c of FIG. 2C is provided with an arc shape metal bulk within the waveguide. By retarding the phase of electric field E1 90xc2x0 by the metal bulk, circularly polarized wave Ec is converted into linearly polarized wave Er. Circular polarizer 53d of FIG. 2D is provided with plate-like metal projections within the waveguide. By retarding the phase of electric field E1 90xc2x0 by the plate-like metal projection, circularly polarized wave Ec is converted into linearly polarized wave Er.
The method of receiving as many channels as possible with one antenna includes the method of receiving the signals of two frequency bands transmitted from one satellite through one antenna, and the method of receiving the signals of two frequency bands transmitted from two satellites located on the same orbit through one antenna. These two different frequency bands correspond to, for example, the C band in the vicinity of 4 GHz and the Ku band in the vicinity of 12 GHz, or an arbitrary combination of frequency bands such as the Ka band in the vicinity of 20 GHz. Two primary radiators are required in order to receive the signals of two frequency bands remote from each other with a parabolic antenna.
The antenna that receives signals of two frequency bands transmitted from the same direction must have directivity with respect to the two frequency bands. In order to provide the same directivity with respect to the signals of two different frequency bands for the parabolic antenna, two primary radiators for the frequency bands must be provided at the focal position of the dish. The same applies for an antenna that carries out transmission and reception at different frequency bands with respect to one satellite.
FIG. 3A is a block diagram showing a schematic structure of a parabolic antenna for a linearly polarized wave where two primary radiators for the frequency bands are provided. This parabolic antenna includes a dish 51 reflecting a linearly polarized wave, a primary radiator 62 for a high frequency band (referred to as fH) receiving the linearly polarized wave collected by dish 51, a primary radiator 63 for a low frequency band (referred to as fL) receiving a linearly polarized wave collected by dish 51, a high frequency band (fH) waveguide 64 transmitting a signal of a high frequency band received by high frequency band (fH) primary radiator 62, and a low frequency band (fL) waveguide 65 transmitting a signal of a low frequency band received by low frequency band (fL) primary radiator 63. fH waveguide 64 and low frequency band (fL) waveguide 65 are formed of the coaxial structure.
FIGS. 3B and 3C are diagrams to describe the electromagnetic mode of high frequency band (fH) waveguide 64 and low frequency band (fL) waveguide 65. Since high frequency band (fH) waveguide 64 is a circular waveguide, the electromagnetic mode within the waveguide corresponds to the TE11 mode of the general circular waveguide, as shown in FIG. 3B. Low frequency band waveguide (fL) 65 is a coaxial waveguide having a conductor (high frequency band waveguide (fH) at the center, so that the electromagnetic mode within the waveguide corresponds to the TE11 mode as shown in FIG. 3C. In the case where a circular polarizer is to be provided at the inner high frequency band waveguide (fH) 64 with respect to a parabolic antenna for a circularly polarized wave, a circular polarizer of any of the structures shown in FIGS. 2A-2D is to be employed within high frequency band (fH) waveguide 64.
FIGS. 4A and 4B correspond to the case where a circular polarizer is provided at the outer fL waveguide 65. A plurality of cylindrical metal projections 82 are provided to have an angle of approximately 45xc2x0 with respect to the linearly polarized wave Er (linearly polarized wave Er to be converted) of the TE11 mode of a coaxial waveguide. Electric field E1 parallel to the plurality of cylindrical metal projections 82 passes cylindrical metal projections 82, whereby the wavelength is reduced. As a result, the phase of electric field E1 is behind the phase of electric field E2 orthogonal to cylindrical metal projections 82. By setting this phase lag to 90xc2x0, the phase difference between electric fields E1 and E2 becomes 0xc2x0. Thus, circularly polarized wave Ec can be converted into a linearly polarized wave Er.
Circular polarizer 81 provided with a plurality of cylindrical metal projections 82 shown in FIGS. 4A and 4B must have the phase and return loss optimized by altering the length of each cylindrical metal projection 82. For this purpose, cylindrical metal projection 82 must be formed of a vis whose length is adjusted one by one in the low frequency band waveguide (fL).
FIG. 5 is a diagram to describe the method of adjusting the length of the, projection in the low frequency band waveguide (fL). As shown in FIG. 5, circular coaxial waveguide converters 92 and 93 are disposed at both sides of circular polarizer 81. The length of cylindrical method projection 82 in the low frequency band waveguide (fL) is adjusted while detecting the phase characteristics of the electric field and the return loss by a vector network analyzer 91.
The phrase characteristics and return loss of the electric field in the direction of E2 shown in FIG. 4A are measured. The phase characteristics refer to the phase lag frequency characteristics from the entrance to the exit of circular polarizer 81. Then, circular polarizer 81 is rotated 90xc2x0, and each projection 82 is inserted in a rotating manner one by one into the waveguide while observing the phase characteristics and the return loss of the electric field in the direction of E1. As each projection 82 is introduced into the waveguide, the phase lag of electric field R1 becomes greater than that of electric field E3, and the return loss of electric field E1 is also deteriorated. There is the case where the return loss becomes favorable by appropriately altering the length of each projection 82 in the waveguide. The length of each projection 82 is to be adjusted to achieve a favorable return loss.
Thus, the length of each projection 82 is adjusted until the phase lag of electric field E1 becomes greater than that of electric field E2 by approximately 90xc2x0 and the return loss of electric field E1 attains a favorable level. Since the phase characteristics and return loss of the electric field in the direction of E2 differs from those of the state prior to the introduction of projection 82 when the length of each projection 82 has been adjusted, circular polarizer 81 is again rotated counterclockwise 90xc2x0 to confirm the phase characteristics and return loss of the electric field in the direction of E2.
An object of the present invention is to provide a circular polarizer that can optimize the phase characteristics and return loss without adjustment.
Another object of the present invention is to provide a circular polarizer of a structure fit for mass production.
According to an aspect of the present invention, a circular polarizer includes a first waveguide, a second waveguide formed in a coaxial structure at the inner side of the first waveguide, and a dielectric member provided to abut against the inner side of the first waveguide and the outer side of the second waveguide, and inclined by approximately 45xc2x0 with respect to a linear plane of polarization.
Since the dielectric member is provided inclined by approximately 45xc2x0 with respect to the linear plane of polarization, the phase lag of the electric field passing through the dielectric member becomes greater than that of the electric field orthogonal to the dielectric member. Therefore, a circularly polarized wave can be converted into a linearly polarized wave. Also, the dielectric member can be formed by a mold to allow the provision of a circular polarizer that is economic and fit for mass production. Adjustment of the phase characteristics and the like is no longer required since the shape of the dielectric member can be determined by experiments.
According to another aspect of the present invention, a circular polarizer includes a first waveguide, a second waveguide formed with a coaxial structure at the inner side of the first waveguide, and a plate-like metal projection provided at the outer side of the second waveguide and inclined by approximately 45xc2x0 with respect to the linear plane of polarization.
Since the plate-like metal projection is provided inclined by approximately 45xc2x0 with respect to the linear plane of polarization, the phase lag of the electric field passing through the plate-like metal projection becomes greater than that of the electric field orthogonal to the plate-like metal projection. Thus, a circularly polarized wave can be converted into a linearly polarized wave. Also, since the plate-like metal projection can be formed with a mold identical to that of the second waveguide, a circular polarizer that is economic and fit for mass production can be provided. Furthermore, adjustment of the phase characteristics and the like is no longer required since the shape of the plate-like metal projection can be determined by experiments.
According to a further aspect of the present invention, a circular polarizer includes a first waveguide, and a second waveguide formed with a coaxial structure at an inner side of the first waveguide, having a cross section in the shape of an ellipse and provided so that the major axis direction of the ellipse has an angle of approximately 45xc2x0 with respect to the linear plane of polarization.
Since the major axis direction of the ellipse is inclined by approximately 45xc2x0 with respect to the linear plane of polarization, the phase lag of the electric field passing through the portion of the major axis direction of the ellipse becomes greater than that of the electric field orthogonal to the major axis direction of the ellipse of the elliptical configuration. Therefore, a circularly polarized wave can be converted into a linearly polarized wave. Also, since the elliptical shape can be formed by a mold identical to that of the second waveguide, a circular polarizer that is economic and fit for mass production can be provided. Furthermore, adjustment of the phase characteristics and the like are not required since the elliptical shape can be determined by experiments.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.